This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which utilize both fundamental and harmonic ultrasonic signals for imaging.
In ultrasonic harmonic imaging, two dimensional (2D) or three dimensional (3D) images are formed by transmitting ultrasound at one frequency (or range of frequencies) and receiving at this frequency and higher harmonics of the transmit frequency. These harmonic signals are generated either by scattering from microbubbles of a harmonic contrast agent as described in U.S. Pat. No. 5,833,613 or by non-linear propagation in tissue (tissue harmonic imaging, or THI) as described in U.S. Pat. No. 5,879,303. Typically, receive beams are formed only from the second harmonic echo signals, with the transmitted (or xe2x80x9cfundamentalxe2x80x9d) echo signals being removed either by filtering or by cancellation techniques such as pulse inversion. See U.S. Pat. No. 5,951,478. For THI, adequate removal of the fundamental signal is essential for the improvements in clutter suppression and contrast resolution which are typically seen.
Under some circumstances it may be of interest to image with both the 2nd harmonic signal and the fundamental signal which would normally be discarded in harmonic imaging. For example, two of the limitations of THI are poor near-field imaging (before non-linear propagation has had a chance to generate a significant 2nd harmonic response) and poor penetration, because the higher frequency 2nd harmonic echo is highly attenuated as compared to the fundamental echo signal. One option for addressing this problem is to image with the fundamental signal in the near and far fields while imaging the 2nd harmonic in the mid-field. See, for example, U.S. Pat. No. 6,283,919 which teaches the formation of ultrasonic images which are a blend of fundamental and harmonic signals. U.S. Pat. No. 6,514,206 describes an ultrasound system and method for doing fundamental and harmonic imaging simultaneously. In the system shown in this patent application a fundamental signal is transmitted from a low end of the transducer passband and harmonic signals are received at an upper end of the passband. Fundamental signals are also sent and received from the center of the transducer passband, the optimal band of the transducer. Images are formed using the received harmonic signals and the optimally centered fundamental signals.
An application which would benefit from a combination of fundamental and harmonic imaging is contrast agent imaging at deep depths within the body. For example, a clinician may be trying to image the vasculature of a tumor deep within the liver. Initially the clinician must locate the tumor so that it can be visually monitored as the contrast agent is applied. This initial search can be conducted at low, fundamental frequencies and at transmit power levels (as indicated by the mechanical index or MI of the transmit beams) which are relatively high for good penetration. When the tumor is captured within the image, the clinician will switch the system to receive in the harmonic mode, and to transmit low MI beams to minimize bubble destruction. However, these changes will often cause the tumor to disappear from sight. This is because the clinician is relying upon the harmonic response of tissue to visualize the tumor prior to administration of the contrast agent, and there is little detectable tissue harmonic response at greater depths for MI""s below 0.2-0.3. Consequently, it would be desirable to be able to conduct such a procedure without loss of visualization of the tumor prior to and while the contrast agent is being administered, but without transmitting high MI beams that would disrupt the microbubbles of the contrast agent.
In accordance with the principles of the present invention, an ultrasonic diagnostic imaging system and method are provided which enable harmonic imaging at low MI""s and at deeper imaging depths. A transmit beam includes two frequency components, a low frequency fundamental component and a high frequency fundamental component which is at approximately the harmonic frequency of the low frequency component. At shallow and intermediate depths the system receives sufficient harmonic energy to image with the harmonic component, but at deeper depths the high frequency fundamental echoes provide signal levels for imaging, particularly in the case of low level transmit signals, as these signals do not suffer from the quadratically diminished signal levels of the nonlinear harmonic echoes. In a preferred embodiment a second pulse is transmitted with a different phase or frequency at the lower fundamental frequency so that the low fundamental frequency components of the returning echoes of the two pulses can be canceled by pulse inversion and the harmonic components reinforced by the two echo receptions.